Rat alpha-LA cDNA and casein cDNA clones have been constructed. Alpha-LA cDNA clone carrying 350 base pair sequence of rat alpha-LA mRNA has been characterized. Using this clone, studies of the organization and the state of methylation of alpha-LA gene sequences during the functional differentiation of rat mammary gland and in various mammary tumors have shown: a) inverse relationship between the expression of the alpha-LA gene and its methylation and b) alpha-LA gene sequences or part of the sequences are reiterated in MTW9 tumor DNA. Analysis of the in vitro synthesized translational products programmed with the poly A plus RNA from the mammary gland and tumors have shown: a) changes in the mRNA activities occurring during the functional differentiation of the mammary gland; b) larger alpha-LA is expressed during pregnancy and both small and large alpha-LA are expressed during lactation and c) MTW9 and MccLx tumor RNA activities are comparable with the gland at early stages of pregnancy except lower alpha-LA mRNA activities are present in these tumors.